With the constant technology improvements, cellular phones comprise many different functions in addition to the common phone function. Thus a user can use his cellular phone to process different functions. For instance, a user can use his phone to take and store pictures, to make and store a video film, to send a text message to another cellular phone, to download mages or music files from an outside device, to register events in a calendar, to listen to mp3 files already downloaded on the memory of the cellular phone from an external device, to listen to the radio, to play to electronic games, etc. In addition, many of these other functions which are supported on a telephone may themselves have devices which support telephone communication. All of these functions are available because the cell phone or other devices comprise many electronic circuits and components that manage these functions.
The different functions of the cellular phone relate to different kinds of data content for example video data, text data or audio data. The transfer of these kinds of data from an outside device to the memory of the cell phone or from the memory of the cell phone to the user occurs through signals carrying the data so the data can be visualized, read or heard. For instance an audio signal carries audio data. A digital signal or an analog signal can represent such an audio signal. Similarly other devices support these and other types of signal.
When a user wants to listen to an mp3 file, the digital audio signal or music signal related to mp3 file data stored in a device such a cellular phone must be transformed or a conversion made before the user can hear the data as an analog audio signal. In fact the data are stored in a digital format and the conversion allows transformation of the said audio signal into an analog signal. Thus the user can hear the signal.
The same conversion occurs when a user receives a phone call from another person. The conversion will convert the digital audio signal coming from another cell phone as soon as this signal reaches the receiving cell phone. In fact the incoming signal is again a digital signal and the user can only hear an analog signal. So the conversion will transform the said digital signal into an analog one.
For both situations, mp3 listening and voice call listening, the conversion of corresponding digital audio signal occurs through an electronic component such as a digital to analog converter (DAC). A digital audio signal having an mp3 source is defined by a wide frequency bandwidth as a wide band signal. A digital audio signal having a voice source is defined by a narrow frequency bandwidth as a narrow band signal. Both these digital audio signals are also defined by their input sample rate or their sampling frequency. The input sample rate of a digital audio signal is typically two times its frequency bandwidth as defined by the Shannon Whittaker sampling theorem for example. A narrow band signal such as a voice signal has a relatively low input sample rate (below about 16 kHz). A wide band signal such as a music signal has a relatively high input sample rate (about 44.1 kHz for standard mp3 files). The electronic circuit of a mobile phone comprises different DACs in order to process such conversions for different kind of data. Sometimes the user may be listening to an mp3 file and then receives a phone call. In this situation, three DACs will realize the conversion from digital signal to analog signal. As the wide band signal representing the music signal is generally a stereo signal, the conversion into a corresponding analog signal uses two DACs. The narrow band telephone call signal uses one DAC. Moreover, one type of DAC is required for wide band signal and another type of DAC is required for narrow band signal.
Therefore this kind of process generates an important current consumption due to the amount of circuitry and the constant battle with expanding battery life. Besides time for developing and manufacturing, the process needs two different kinds of DACs. So the whole system of the cell phone including the different kinds of DACS takes much more time than would otherwise be the case.
FIG. 1 shows a prior art schematic structure of a circuit 100 for a mobile phone. This circuit 100 processes the conversion of digital audio signals into analog audio signals. This circuit comprises three inputs for three signals. Each signal comprises an amplitude which determines the instantaneous intensity or the average intensity of the signal. Each signal also comprises its own frequency which is different from the sampling frequency. Each signal comprises bits that are serial bits. The input 102 relates to a narrow band digital audio signal 104 such as voice signals with a given, usually low, input sample rate. This voice signal 104 relates to a phone call which the user receives on a mobile phone. A voice signal usually comprises a 13-bits or 14-bits coded structure. This means that all 13-bits or 14-bits belong to one signal. The inputs 106 and 108 relate to wide band audio signals 110 and 112. The sample rate of these signals 110 and 112 differ from the sample rate of the signal 104. These wide band signals represent a music signal. The combination of these two signals 110 and 112 provides a stereo music signal. A music signal usually comprises a 16-bits (or more) coded structure. This means that 16 bits belong to one signal. A music signal relates for instance to a signal corresponding to an mp3 file already registered on storage means of the mobile phone for instance. In the prior art situations, connection lines 114, 116 and 118 are dedicated for each of the three signals 104, 110 and 112. The connection lines each comprise one serial parallel interface or interface module 120 and one digital analog converter (DAC) 122. The interface 120 transforms all the serial bits into parallel bits. Concerning the voice signal 104, the interface module 120 transforms the 13-bits or 14-bits signal 104 into 13 signals or 14 signals with a 1-bit coded structure. In the same way, concerning the music signals 110 and 112, the interface module 120 transforms the 16-bits coded structure into 16 signals with a 1-bit coded structure. This digital analog converter allows the conversion of a digital signal to a corresponding analog signal. The DAC 122 comprises a digital filter 124, a sigma delta modulator 126, a D-to-A filter 128 and a smoothing filter.
In the prior art, U.S. Pat. No. 6,714,825 describes a multi-channel reproducing method in order to convert multi-channel audio sources having different sample rates. This method employs less DACs than the number of incoming channels. However this method requires a specific sampling rate conversion in order to convert all the different signals to obtain the same bandwidth for all the signals. Also this process increases the digital complexity of the circuit.
It appears that if a user wants to listen simultaneously to voice call signals and music signals on a device such as a mobile phone, solutions exist but they necessitate a costly hardware implementation as described above. A number of different methods have been proposed to overcome the problem of reducing the number of DAC in a mobile device but these solutions are not very efficient.
An object of the present invention is to provide a method and an apparatus which overcome at least some of the problems associated with the prior art.